Elgato Live Preview

The Elgato Game Capture HD application (GCHD) for macOS allows the user to get a low-latency live-preview (called "Instant Gameview") of the content being captured by the Elgato HD60S. Unfortunately (as of version 2.6.1), the framerate of this preview is quite choppy, making it barely usable for those who intend to play directly within this view.

This project aims to fix this issue by providing an external app that display the uncompressed frames captured by the device. This app offers two advantages compared to the official live-preview :

• No framedrops
• Improved image quality when the image is stretched (e.g. on a 5K iMac screen) thanks to a sharpen filter

As a side-effect, this project allows to access uncompressed video-frames from the GCHD application. It means that third-party software like OBS Studio can use this to capture HD60S video with no extra latency or compression. As a proof-of-concept, this repository hosts an OBS plugin that does exactly that.

How it works

This project is based on the observation that while the preview is choppy, the recorded videos have stable 60fps. It means that the video encoder used by the GCHD application gets uncompressed frames at a stable framerate.

This project is made of 3 parts :

• x264_hook: a library that intercepts calls made to the x264 library (software video encoder used by GCHD). To achieve this, it uses the dyld_interposing feature from the DYLD loader on macOS. With this, it retrieves uncompressed YUV420P frames sent to the x264_encoder_encode() function. These frames are made available to external softwares through a Unix domain socket (located at /tmp/elgato_raw_frames)
• live_preview: an OpenGL application that receives uncompressed frames from the socket and displays them. This application also applies a sharpen filter on the displayed frames to improve image quality when the image is stretched.
• obs_plugin: a simple OBS plugin that receives uncompressed frames from the socket and displays them in a layer of OBS studio. Note that this is video only, no audio is captured (though a similar approach is probably possible for audio). Disclaimer: This plugin is a working proof-of-concept that has been tested with OBS Studio 19.0.3 only. I have no plan to maintain it as I do not use OBS personally.

How to run

Live Preview

To run the live-preview, you first need to launch the GCHD application with the x264 hook activated. The easiest way to do that is to open a terminal and to launch the startGameCaptureHD.sh script:

cd elgato-live-preview
./startGameCaptureHD.sh

It assumes that the GCHD application is installed at its default path (/Applications//Applications/Game\ Capture\ HD.app). In order to intercept software encoder functions, hardware encoding has to be disabled in the HD60S settings from GCHD. If everything went well, you should see the following line in the terminal: Frame-server started successfully.

Now, you can run the live-preview application by opening a terminal and launching the elgato_live_preview executable:

cd elgato-live-preview
./elgato_live_preview

Note: To save CPU power, software encoding is actually disabled by x264_hook library. If you want still want software encoding in the GCHD application while using elgato_live_preview, you can start GHCD with the following command: X264_HOOK_ENCODE_ENABLED=1 ./startGameCaptureHD.sh

OBS Plugin

To display HD60S frames inside OBS Studio, you first need to install the plugin. To do so, copy obs-elgato.so to the plugins directory of OBS Studio (/Applications/OBS.app/Contents/Resources/obs-plugins if OBS is installed in /Applications).

Then, launch the startGameCaptureHD.sh script (cf. previous section) and launch OBS Studio. In OBS Studio, create a new Elgago HD60S source and that's it.

How to build

Dependencies

To build this project, you'll need:

• XCode: can be installed from the AppStore
• CMake >=3.0: can be installed with brew install cmake (cf. Brew website)
• GLFW >=3.2: can be installed with brew install glfw (cf. Brew website)

How to build x264_hook

From a terminal:

cd elgato-live-preview/x264_hook
mkdir build
cd build
cmake ..
make

The libelgato_x264_hook.dylib and startGameCaptureHD.sh will be generated.

How to build live_preview

From a terminal:

cd elgato-live-preview/live_preview
mkdir build
cd build
cmake ..
make

The elgato_live_preview executable will be generated.

How to build obs_plugin

• Clone the OBS studio repository
• Create a soft-link of the obs_plugin directory to obs-studio/plugins/obs-elgato
• Add an entry for obs-elgato in obs-studio/plugins/CMakeLists.txt
• Build OBS Studio again

What's next

• Intercept calls made to the hardware encoder (VideoToolbox framework)
• Make the sharpen filter customizable